The technology for reducing the open-end portion of a closed end container (necking) has been in existence for over one hundred years. The procedure was originally developed for artillery shells, with a larger diameter shell casing being reduced to retain a smaller diameter projectile. The process by which this is accomplished today is called die necking. The basic concept of necking is to force a typically cylindrical, thin walled metal body or shell at a given diameter and physically push it into a die or series of progressively smaller dies. In this process a reduction in diameter of the open end is realized.
In metal food and beverage cans, the primary purpose for a reduced diameter at the open-end is material savings, and thus realized as cost savings. Because the end panel is of a thickness that is on the order of at least twice the thickness of a typical sidewall, as the diameter of the container is reduced, the amount of material necessary for the end panel is reduced by a greater amount. In certain applications such as aerosol containers, the necking operation is performed to bring the opening to a specific diameter to facilitate a standard size valve assembly and eliminate any secondary adaptor that would otherwise be necessary. A secondary consideration of reducing the diameter of the end of the container is the reduction in the longitudinal stress exerted on the end of the container. As the cap size is reduced, this stress is reduced and the thickness requirement of the end cap is also reduced. The third consideration for diameter reduction is visual. Many aesthetically pleasing shapes can be achieved by necking conventional cylindrical block shapes into tapered geometries and containers that resemble bottles.
There are practical limits on the reduction of the diameter of the material for any given material in any given die. The strength of the can body depends on a number of factors including the Young's modulus and yield stress of the material, the plate thickness and the can diameter. If the practical limit on diameter reduction is exceeded, the material will wrinkle, pleat, pucker or tear at a point inherent to the geometrical characteristics and type of metal being necked.
Conventional die necking of metal containers is accomplished with large-scale machinery that is very difficult to develop the fine tuned properties required to manufacture containers with significant neck length. The development of necking profiles is currently a long, involved, trial and error process that can take months to establish the proper parameters for each necking stage necessary to produce longneck containers. Specifically, current die necker technology uses hard cams to provide motion to pusher and knock-out rams. Key parameters such as cam profile and cam throw must be tested and tweaked with each incremental change in the necking profile. Each time a change is made, the machine must be taken down and modified in a lengthy process to redesign and refit the new cams.
U.S. Pat. No. 5,355,710 discloses a conventional method and apparatus for necking a metal container. The disclosure of this patent is specifically incorporated herein by reference.